Utilization of renewable raw materials for the production of value added chemicals is one of the main R&D activities all over the world to overcome the dependence on depleting fossil fuels. Lactic acid is one of the renewable chemical obtained from fermentation of biomass and is being considered as one of the important raw material for the production of value added chemicals. Further, acrylic acid and acrylates comprise a class of materials having potential industrial applications especially in the field of polymers and co polymers. The relatively high cost of acrylates as a result of the methods of preparation available in the prior art and other inefficiencies limits the use of this important material.
Catalytic dehydration of lactic acid to acrylic acid is being investigated in both academic and industrial laboratories with limited success. When heated at moderate temperatures, lactic acid readily converts to lactides or polylactic acid. At higher temperatures, lactic acid decompose to acetaldehyde, carbon dioxide and water, a property typical of alpha hydroxy acids.
Heterogeneous catalysts have been used by scientific community to bring about catalytic dehydration of lactic acid to acrylic acid however, low conversion of lactic acid and lower selectivity for acrylic acid as well as catalyst deactivation highlight the main drawbacks in their use.
U.S. Pat. No. 4,786,756 describes a process for the catalytic conversion of lactic acid and/or ammonium lactate to acrylic acid which comprises contacting a mixture of water and lactic acid and/or ammonium lactate in the vapor phase with solid aluminum phosphate which has been treated with an aqueous inorganic base and calcined at a temperature in the range from 300° C. to 650° C. The base is selected from aqueous ammonium hydroxide and potassium hydroxide. With lactic acid as the feed, the acrylic acid yield of 43.3% has been reported at 340° C.
U.S. Pat. No. 4,729,978 discloses inert metal oxide selected from the group consisting of silica, titania and the alumina phosphate salt is selected from the group consisting of NaH2PO4, Na2HPO4, K2HPO4, KH2PO4, Li2HPO4, LiH2PO4, LaPO4, Mg3(PO4)2 and Ca(H2PO4)2. Further, the base is selected from the group consisting of Na2CO3, NaHCO3, KHCO3, K2CO3, LiCO3, CaCO3, MgCO3 and La(CO3)3 for conversion of lactic acid to acrylic acid. With Lactic acid as the feed, acrylic acid yield of 58% with a selectivity of 65% has been claimed.
Efficient dehydration of methyl lactate to acrylic acid using Ca3(PO4)2—SiO2 catalyst by Jong-Min Lee et. al (Catalysis Communications, Volume 11, Issue 15, 25 Sep. 2010, Pages 1176-1180) describes a series of catalysts consisting of Ca3(PO4)2 supported on SiO2 (silicate, colloidal silica and fumed silica) and Ca3(PO4)2—SiO2 (silicate) with different loadings of Ca3(PO4)2 (70 to 95 wt. %) was prepared by sol-gel and wet-impregnation methods. All the catalysts were found to be active in the vapor phase dehydration of methyl lactate (ML) to give mainly acrylic acid (AA), methyl acrylate (MA). Among the catalysts, Ca3(PO4)2—SiO2(silicate) of (80:20 wt. %) was found to be an efficient catalyst in the dehydration of Methyl Lactate, which gave 73.6% conversion of Methyl Lactate with selectivity for Acrylic Acid and Methyl Acrylate together (77.1%).
U.S. Pat. No. 2,859,240 discloses the catalytic production of acrylic acid from lactic acid which comprises the steps of (1) bringing lactic acid into contact with a dehydration catalyst consisting essentially of at least one member of the group consisting of the sulfates and phosphates of metals of groups I and II which are at least-as high as cadmium in the electromotive series, at a temperature within the range of 200° C. to 600° C. and (2) separating acrylic acid from at least some of the reaction products. It further discloses that the salts of the alkaline earth metals, especially of calcium, strontium and barium provide the highest yields and are preferred. The practice of this invention is not limited to fixed bed catalysts, but may be carried out with a fluidized bed when such conditions are advantageous. Furthermore, in example 32 granules of a mixture of tricalcium phosphate and sodium pyrophosphate Na4P2O7 in-25:1 molar ratio were employed as the catalyst mass to convert lactic acid to acrylic acid with 48-52% yield.
References may be made to Engineering Science Paper titled “Research on microwave assisted Dehydration of Lactic acid to Acrylic Acid” dated 25 Feb. 2012 discloses the use of disodium hydrogen phosphate and potassium hydrogen phosphate as promoter along with Calcium sulphate and copper sulphate.
There, however, remains a need to improve upon the existing catalytic process of conversion of lactic acid to acrylic acid which can result in 100% conversion of lactic acid and is highly selective to acrylic acid with minimum production of acetaldehyde and other products.